1. Field of the Invention
The present invention relates to optical matrix switches made by single layer liquid crystal switchable elements and the fabrication methods.
2. Description of the Related Art
High performance optical matrix switches are critical components for telecom. Quality switchable mirrors are important in constructing the high performance optical matrix switches. In the following, related prior art technologies are cited as the reference for the present invention. The first prior art technology is based on liquid crystal phase shifter/rotator as disclosed by, Corning, Exhibition in NFOEC'2000, Denver, Colo., Aug. 27–31, 2000 and SpectraSwitch, Exhibition in NFOEC'2000, Denver, Colo., Aug. 27–Aug. 31, 2000 and Chorum, Exhibition in NFOEC'2000, Denver, Colo., Aug. 27–Aug. 31, 2000. This technology switches light without involving moving parts and is relatively fast. FIG. 1 shows the schematic diagram of the switch which is copied from the SpectraSwitch website.
In order to accomplish one switching action of an unpolarized beam of light, numerous optical components have to be used. The light has to pass a passive polarizing beam splitter to be split into two beams with orthogonal polarization states. Each split beam has to pass an active liquid crystal phase retarder/rotator so that their polarization is modulated. Next, each beam has to be reflected by a mirror. Finally, the two beams pass another polarizing beam splitter where they are combined to become an unpolarized beam again. Modulation of the liquid crystal phase retarder/rotator determines the output direction of the final beam from the second beam splitter. Due to the fact that six optical elements (two beam splitters, two LC modulators and two mirrors) are involved in accomplishing one switching action, this is no longer a single layer switchable mirror. Furthermore, the optical modulation by the liquid crystal phase shifter or rotator is intrinsically not broadband, which imposes significant optical cross talk at the wavelengths outside the liquid crystal device bandwidth. Finally, this technology intrinsically is polarization dependent and has significant polarization dispersion loss (PDL).
The second prior art technology for a motionless electrically switchable mirror is holographically-formed polymer dispersed liquid crystal, or H-PDLC. H-PDLC's as disclosed by V. Natarajan, R. L. Sutherland, V. P. Tondiglia, T. J. Bunning, and W. W. Adams, Jounal of Non-Linear Optical Physics and Materials, 89 (1996) and G. P. Crawford, T. G. Fiske, and L. D. Silverstein, Journal of the SID, 45 (1997) are a variant of polymer dispersed liquid crystals (PDLCs) formed under holographic conditions. Although H-PDLC features single layer, motionless, and polarization insensitive, it has a limited spectral bandwidth (only around 20 nm) and limited reflection efficiency. FIG. 2 illustrates the H-PDLC structure, wherein the LC structure 205, 210 are the two states of operation. The holography introduces a periodic array of liquid crystal droplets (with diameters of 20 nm–200 nm) and solid polymer planes with an interference fringe spacing d. As shown by the LC structure 205, when no voltage applied, the LC directors 207 are misaligned. Since the refractive index of the LC droplets (nLC) is different from that of the polymer planes (np), light 208 is scattered at the droplet interface. Due to the periodic modulation in the refractive index, light at the Bragg wavelength is reflected back to the observer. However, as shown in LCD structure 210, when a voltage is applied, the liquid crystals inside the droplets are aligned along the field direction, assuming that the LC has a positive dielectric anisotropy. Therefore, the liquid crystal index becomes nlco. If nlco is equal to np, the periodic refractive index modulation disappears and the incident light is transmitted, realizing the switching of the device.
The third prior art technology is a modified H-PDLC technology based on cholesteric liquid crystal composite materials consisting of polymeric and non-polymeric liquid crystal compounds together with chiral additives as disclosed by M. Date and T. Hisaki, “Helical Aligned holographic polymer dispersed liquid crystal (HPKLC)”, Asia Display'2000, Japan. The device was fabricated by putting liquid crystal mixture into the space between two glass plates with anti-parallel rubbed polyimide layers and irradiating it with interferential fringes of laser light. The helical pitch was much shorter than the spacing. The laser radiation formed a polymer network periodically corresponding to the interferential fringes since polymerization occurs at fringe peaks. It has been claimed that the device was transparent under zero voltage (reverse mode). When an electric field was applied, a dip in the transmission spectrum was observed that corresponds to the diffraction. It was further claimed that the device was polarization independent. However, the device shows a very low reflectivity (only around 50% or less) and its spectral bandwidth is narrow (only around 30 nm).
The forth prior art technology is disclosed in the U.S. Pat. No. 6,133,971 by Silverstein et al for constructing a holographically formed reflective display comprising a plurality of anisotropic polymers sheets, formed from at least a photo-active monomer and a photo initiator, that separate a liquid crystal material into a plurality of liquid crystal material regions. The resultant device reflects at least one selective wavelength of light with a narrow bandwidth. It is claimed that both nematic liquid crystal and/or cholesteric liquid crystal can be used to make the device.
The fifth prior art technology for constructing a switchable liquid crystal mirror is based on cholesteric liquid crystal. Conventional CLC is narrow band and polarization sensitive. FIGS. 3(a) and 3(c) show how conventional CLC's 300, when in a planar alignment which, adopt a spiral arrangement 305 to form a uniform helical structure with a pitch “P”. Such a helical structure results in a light reflection at a center wavelength λo=naP with a natural bandwidth given Δλi=Δn P where “P” is the helix pitch. The bandwidth Δλ is mainly determined by the CLC birefringence Δn. 50% of the unpolarized incident light within the band is reflected into a circular polarization state that has the same handedness as the CLC spiral, while the remaining 50% is transmitted with the opposite polarization state. The light outside the bandwidth will pass the CLC regardless of its polarization state. Depending on the CLC material, the narrow band polarizing state can be either electro-optically passive (non-switchable) if the CLC is polymerized to form a solid film (see FIG. 3 (a) and (b)). Or, it can be electro-optically active (switchable) if the CLC is made from non-polymeric low molecular weight (LMW) liquid crystal. Usually, the switchable CLC is in a planar texture under zero voltage (see FIG. 3 (a) and (b)). When it is switched by an electric field, all the CLC molecules are untwisted and aligned along the field direction so that it becomes transparent (see FIG. 3 (c)).
In order to use conventional CLC to make a switchable mirror for reflecting un-polarized light, two pieces of CLCs with opposite handedness are needed to form a double layer configuration. The reflected beam comprises two partially overlapped beams with opposite circular polarizations, one being reflected from the first CLC while the second beam from the second CLC, as shown in FIG. 4. The displacement of the two reflected beams becomes more pronounced when the incoming light is incident at a large angle, which makes this apparatus less desirable for building a switch for optical communication than the other aforementioned apparatuss.
The sixth prior art technology is based on an improved CLC material that gives rise to a broad spectral bandwidth. However, it still needs two layers of CLC's with opposite handedness for operation under an unpolarized light.
The seventh prior art technology is disclosed by, J. E. Fouquet, “Compact optical cross-connect switch based on total internal reflection in a fluid-containing planar lightwave circuit”, OFC'2000, Baltimore, Md., Mar. 5–10, 2000, which is based on a fully integrated optical waveguide in which a special liquid is filled. At the bottom of each intersection of two waveguides, there is a “micro-thermal boiler”. If a switch action is desired at the spot, the “boiler” heats the liquid to the boiling point so that air bubble is created. The generated air bubble behaves like a mirror that reflects light beam into the desired waveguide channel. This technology induces flow of the fluid therefore it is no longer motionless.
The eighth prior art technology is based on micro-mirror which is fabricated via micro-electrical mechanical system (MEMS) technology as disclosed by, JDS-U, Exhibition in NFOEC'2000, Denver, Colo., Aug. 27–Aug. 31, 2000 and Nortel, Exhibition in NFOEC'2000, Denver, Colo., Aug. 27–Aug. 31, 2000. Switching of light beam is realized via tilting or swinging the micro-mirror.
The final, but not the last, prior art technology is based on thermal-electric technology that switches light via thermally induced index change as disclosed by, Mitsuhiro makilara, Fusao Shimokawa, and Kazumasa Kaneko, “Strictly Non-Blocking N×N Thermo-Capillarity Optical Matrix Switch using Silica-based Waveguide”, OFC'2000, Baltimore, Md., Mar. 5–10, 2000.
In summary, there does not exist a technology that promises an electrically switchable mirror that features single layer, motionless, broad spectral bandwidth, and high efficiency.